Product areas in ideation in sub forums

I created a sub forum, as I only want a selected group of community members to be able to post their ideations there. Unfortunately product areas can’t initially be selected there, if one tries to submit the idea.
If you however switch from the idea to question or conversation you can use the product areas, BUT than your ideation will directly be posted to the other ideas and no longer in the sub forum.

Is there any way to either:
Restrict who can post ideas in the normal ideation section (I think there isn’t as you can’t decide rather between the rights to create, view and vote, but only allow all three of those) or create new product areas, that can be used in the sub forum?